This relates generally to integrated circuits and more particularly, to integrated circuits with phase-locked loops.
A phase-locked loop is commonly used in radio, telecommunications, computers, and other electronic applications to generate an output clock signal whose phase is related or “locked” to the phase of an input clock signal. In general, a phase-locked loop (PLL) includes a phase detector and a voltage-controlled oscillator. The oscillator generates a periodic signal. The phase detector compares the phase of that periodic signal with the phase of the input clock signal and then adjusts the oscillator to keep the phases in sync. Operated in this way, the phase-locked loop also keeps the frequencies of the input and output clock signals matched or at least proportional to one another.
A phase-locked loop generally has limited control over the availability of the input clock source. For example, the input clock might stop toggling after some time due to latency issues or some unintentional clock gating. For a charge-pump-based PPL (i.e., a PLL that uses a charge pump to directly control the voltage-controlled oscillator), this clock loss event could present a problem when the charge pump continues to charge or discharge beyond a desired operating voltage range. Under such scenarios, the voltage-controlled oscillator, which is typically designed using thin-gate oxide transistors, will be susceptible to electrical overstress.
In an effort to improve reliability and to protect against this type of electrical overstress, voltage-controlled oscillators are sometimes designed using native thick-gate oxide transistors with zero threshold voltage. Thick-gate oxide transistors with zero threshold voltage are able to withstand high levels of electrical overstress while maintaining operation in the saturation mode.
However, it may not always be possible or feasible to include thick-gate oxide devices on an integrated circuit. Forming thick-gate oxide transistors requires additional processing steps at the foundry and increases cost. It is within this context that the embodiments herein arise.